#!/usr/bin/perl -w
# rename all files with a certain extention in a directory

use strict;

my $dirname;
$dirname = '/substitue_your_path_here';

opendir(DIR, $dirname) or die "Can't opendir $dirname: $!";

while ( defined (my $file = readdir DIR) ) {
        # ignore current and parent directory . and ..
        next if $file =~ /^\.\.?$/;
        my $new = $file;
        # substitute new file extension
        $new =~ s/\.old_extension$/\.new_extension/;
        rename($file,$new)
}
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Reference Perl Cookbook Chapter 9 and Programming Perl Chapter 29