Given string1 and string2 as command line arguments, the snippet will return a unique list of characters both strings have in the order they appear in the second string.
#!/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 //, $str
+2;
}

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In reply to Unique List of Common Characters Between Two Strings by Limbic~Region

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