There you go: the idea is to store for each filename in which paths it occurs.

#!/usr/bin/perl
use warnings;
use strict;
use File::Find;
use File::Spec;
  
  
my $directory = shift @ARGV || '/mnt/music/very-good';
  
find (\&wanted, $directory);

my %path_file;

sub wanted {
    my $path = $File::Find::dir;
    my $filename = File::Spec->abs2rel($File::Find::name, $path);

    push @{$path_file{$filename}}, $path;

}

while (my ($filename, $paths) = each %path_file){
    if (scalar @$paths >= 2){
        print "$filename occurs in these paths: ", join(", ", @$paths)
+, "\n";
    }
#    else {
#        print "$filename is uniq\n";
#    }
}
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