Instead of

#I create my file list with 
my (@filelist) = @ARGV;
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It might be better to call the program as $mp3concat -s 700 `pwd` and then use:

my $mp3dir=shift or do { 
    warn "No directory specified, using current directory.\n" ;
    $mp3dir='.';
    };
opendir(MPD,$mp3dir) or die("can't open directory, $mp3dir.\n $!");
my @filelist=readdir(MPD);
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Remember not to use "." and "..", though.