Audit a Legacy Perl Script for Perl 5.24 Compatibility

You will finish with a small, repeatable compatibility check for a Perl script that may be moving from 5.22 to 5.24 or a later interpreter. The check identifies source constructs that became errors, exercises a few changed behaviours, and leaves the original script untouched.

Allow about 20 minutes for one script, longer for a program with many modules. You need a shell and Perl. The installed interpreter used for these examples is Perl 5.38.2 from Ubuntu package perl 5.38.2-3.2ubuntu0.6. That is newer than the release described by perl5240delta(1), so use its results as a modern confirmation of changes first introduced in 5.24, not as proof that every dependency supports your target Perl.

1. Record the interpreter you are testing

Start with ordinary, read-only commands. The version matters because the delta document describes the change from Perl 5.22.0 to 5.24.0, while the executable on this machine is newer:

$ perl -e 'printf "perl %vd\n", $^V'
perl 5.38.2
$ dpkg-query -W -f='${Package} ${Version}\n' perl perl-base perl-doc
perl 5.38.2-3.2ubuntu0.6
perl-base 5.38.2-3.2ubuntu0.6
perl-doc 5.38.2-3.2ubuntu0.6

If you need to certify a deployment against exactly 5.24, run the same checks with that interpreter in an isolated test environment. Do not replace the system Perl merely to reproduce an old compatibility issue.

2. Read the relevant release boundary

The local manual is a change list, not a migration tool. For an audit, concentrate on the sections headed Incompatible Changes, Deprecations and Security. The most likely source-level blockers are:

• my $_ no longer compiles.
• The experimental autoderef feature was removed.
• /\C/ became a compile-time error.
• chdir('') and chdir(undef) no longer change to the home directory.
• Some invalid regular-expression properties fail at compilation instead of later at runtime.

The same document also records useful intentional changes: postfix dereferencing is no longer experimental, Unicode 8.0 is supported, \b{lb} is available, and negative or over-sized integer shifts have defined behaviour. Treat those as things to test, not as reasons to change working code.

3. Scan source without starting the application

Search the working tree for the constructs that are explicitly incompatible. This is an ordinary command and needs no elevated privileges:

$ rg -n --glob '*.pl' --glob '*.pm' \
    'my[[:space:]]+\$_|autoderef|\\C|chdir[[:space:]]*\([[:space:]]*(undef|""|\x27\x27)' \
    /path/to/project

Adjust the glob list for your project. Search results are leads, not automatic fixes: a string in documentation or a quoted regular expression may not be executable Perl. Conversely, generated code or a module loaded from another directory may be outside the first search.

Now ask Perl to compile the entry point:

$ perl -c /path/to/project/bin/legacy-tool
/path/to/project/bin/legacy-tool syntax OK

-c checks syntax and then exits, but Perl still runs compile-time blocks such as BEGIN. Use a disposable test environment if the program imports modules with side effects. Do not treat a successful compile as a functional or security review.

4. Confirm the removed constructs fail where expected

These probes do not touch files or services. They are useful for confirming that the interpreter used by a test job enforces the boundary:

$ perl -e 'my $_;'
Can't use global $_ in "my" at -e line 1, near "my $_"
Execution of -e aborted due to compilation errors.
$ perl -e 'use feature "autoderef";'
Feature "autoderef" is not supported by Perl 5.38.2 at -e line 1.
BEGIN failed, compilation aborted.
$ perl -e '"x" =~ /\C/;'
\C no longer supported in regex; marked by <-- HERE in m/\ <-- HERE C/ at -e line 1.

Exact diagnostics can vary between Perl releases and warning settings. The useful result is a non-zero status and a compile-time failure for code that still relies on a removed feature. The \C diagnostic may be phrased differently by a newer interpreter, so inspect both status and stderr rather than matching one complete sentence in a test.

For chdir, do not run a migration probe in a directory whose current location matters. Make the intended destination explicit in application code with chdir(), or check the return value and handle failure. The 5.24 change removes an unsafe implicit home-directory fallback.

5. Exercise changed behaviour with harmless expressions

Some 5.24 changes are semantics rather than syntax. Test them with values and output that do not depend on application data. Reordered precision arguments, for example, are supported in 5.24:

$ perl -e 'print sprintf(q{|%.*2$d|}, 2, 3), "\n"'
|002|

Postfix dereferencing is likewise usable without an experimental feature declaration:

$ perl -e 'my $values = [10, 20]; print $values->[1], "\n"'
20

Integer shifts are constrained by the native integer width. Check the width before relying on a boundary case:

$ perl -MConfig -e 'print $Config{uvsize} * 8, "\n"'
64
$ perl -e 'printf "%d %d\n", 1 << -1, 1 << 1000'
0 0

A negative shift reverses direction, and shifting by the native width or more produces zero in these non-negative cases. If your program needs wider arithmetic, test the bigint pragma rather than assuming the machine word grows with the input.

6. Separate source fixes from deployment checks

Fix each confirmed incompatibility in a branch or disposable copy, then repeat the source search and compile check. Keep a short record of the file, the old construct, the replacement and the test that covers it. Do not silence warnings globally just to make an upgrade green.

The release notes also describe security fixes, including safer temporary-file permissions and removal of duplicate environment entries before child processes are started. Those are interpreter improvements, not substitutes for safe application code. Do not add sudo to a Perl test unless the script genuinely requires a privileged resource. Running an untrusted or unfamiliar script as root can turn a compatibility check into a system change.

If a compile check fails after a change, undo only that change with your normal version-control workflow, or restore the disposable copy. If a test created files or changed a service, stop and use the project's documented recovery procedure before continuing. The probes in this guide create neither persistent files nor service state.