#!/usr/bin/perl
use strict;
use warnings;

my ($str1, $str2) = @ARGV;
die "Usage: $0 <string1> <string2>" if ! defined $str2;

print join '', In_Common($str1, $str2);

sub In_Common {
    my ($str1, $str2) = @_;
    my %u;
    $u{ $_ } = 1 for split //, $str1;
    return grep { $u{ $_ } ? ( $u{ $_ }-- , $_ ) : () } split //, $str2;
}