after all, i still don't understand, why you're usiińg substr() at all - instead of a regex:
#!/usr/bin/perl -w
use strict;
my $str = "This_is_a_song_.mp3";
$str =~ s/_(\.mp3)$/$1/g;
print "$str\n";

[download]
another way might be: 
use strict;
my $str = "This_is_a_song_.mp3";
my ($pre, $post) = split /\./, $str;
chop $pre;
$str = "$pre.$post";
print "$str\n";

[download]
... and so on, and so on... :D

language is a virus from outer space.

In reply to Re: Using substr to remove characters by thcsoft
in thread Using substr to remove characters by JediWizard

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