<code>
use strict;
use warnings;

my @files = (
   '/a/b/12.jpg',
   '/a/b/c/d/12.jpg',
   '/a/b/c/12.jpg',
   '/a/b/e/12.jpg',
   '/a/b/c/f/12.jpg',
   '/a/b/e/12.jpg',
   '/a/b/12.jpg',
   '/b/12.jpg',
   );

my $cp = 0;
foreach my $file (@files) {
	$file =~ m#(.*/)#;
	$cp = $1 if $cp eq '0'; # only for the first run
	$cp = $1 if (length($cp) > length($1));
	print "$1 $file\n";
}

print "Common Path is $cp\n";
</code>
<code>
use strict;
use warnings;
use Data::Dumper;

my $h = {
  '/' => 
    {a => [12, 13],
    b => [14]},
  };
  
print Dumper $h;
</code>