When you have a problem, don't hide the errors! Use use strict; and use warnings;.

$ perl -Mstrict -wle'my @f = glob("*.mp3"); push @{$f[/ad/]}, $_ forea
+ch @f;'
Can't use string ("bar.mp3") as an ARRAY ref while "strict refs" in us
+e at -e line 1.
[download]

In the first snippet, $f[0] and $f[1] contain undef or an array reference. push @{$f[...]}, pushes unto the referenced array (creating it if necessary).

In the second snippet, $f[0] and $f[1] contain file names. push @{$f[...]}, uses the file name as an array name and push unto the named array.

$ perl -le '@f = glob("*.mp3"); push @{$f[/ad/]}, $_ foreach @f; use D
+ata::Dumper; print Dumper(\@f, \@{"bad.mp3"}, \@{"bar.mp3"})'
$VAR1 = [
          'bad.mp3',
          'bar.mp3',
          'foo.mp3',
          'moo.mp3'
        ];
$VAR2 = [
          'bar.mp3',
          'foo.mp3',
          'moo.mp3'
        ];
$VAR3 = [
          'bad.mp3'
        ];
[download]

Perhaps you shouldn't use the same array to save your file names and your arrays of file names...