use strict;
use warnings;

open(my $fh, "<", "input.txt") 
  or die "cannot open < input.txt: $!";

my %duplicates;
while (my $line = <$fh>) {
       chomp $line;
       my ($fname, $fext) = split(' ',$line);
       $duplicates{$fname}{$fext}++;
}

##</code><code>##

/foo/bar/baz/123 aaa
/foo/bar/baz/123 aab
/foo/bar/baz/123 aac
/foo/bar/baz/124 aaa
/foo/bar/baz/124 aab