I hope that you're not being told to port to older versions that you don't have access to -- the task would be a bit too speculative if you can't test the scripts against particular older perl releases that you can still run yourself.

Assuming that you only need to port to versions that you have on hand, the obvious starting point is to make a list of the available relevant versions, remove the initial "shebang" line from your scripts, and run them through the older versions like this:

/path/to/perl5.006/bin/perl -cw script1 > script1.5.006.errs 2>&1
/path/to/perl5.005/bin/perl -cw script1 > script1.5.005.errs 2>&1
etc.
[download]

Store each version's error report in a separate file, then take the union of the messages in all files to assess what damage needs to be done to each script. Or it might be better to work on the intersections -- where multiple versions report the same problem -- fix those first before altering lines that only cause a problem in one version, or that cause different error reports in different versions.

Don't do any more research on perl version lore than you have to -- just check the line numbers of the error messages, and grep through the perldelta docs for relevant hints based on the errors.

Bear in mind that some errors may be due to extra modules that are present in your up-to-date perl release but are not installed for the older versions. Adding those modules to the older versions might be tricky (esp. if you have to find older versions of the modules :P).

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