perl -c works under 5.004 I think - identify each type of error with this and make a manual change e.g.

open my $fh, "<file.txt" or die "blah $!\n";
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produces with perl -c (can run perl -c on anything including .pm files)

Can't use an undefined value as filehandle reference at blah.pl line 1
+23
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So you resolve it with...

use FileHandle

# ...

my $fh = new FileHandle();
$fh -> open( "< file.txt" ) or die "blah $!\n";
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It shouldn't take too long -- if there are too many, write a converter that implicitly splits by assigning $/ = ';', pattern-matches for each issue and does the conversion - it's not much work actually.

You'll also have to read release notes of intervening versions.