#! perl -slw
use strict;

sub common {
    die 'Bad args' unless @_ == 2;
    my $output = $_[ 0 ] ^ $_[ 1 ];

    $output =~ s[([^\0]+|[\0]+)]{
        my $p = $-[0];
        my $l = $+[0] - $-[ 0 ];
        ord( $1 ) == 0
            ? substr( $_[ 0 ], $p, $l )
            : join '', map{ '[' . substr( $_, $p, $l ) . ']' } @_
    }ge;
    return $output;
}

while( my $str1 = <DATA> ) {
    my $str2 = <DATA>;
    print common( $str1, $str2 );
}

__END__
a11211b123
a11222b123
The quick brown fox jumps over the lazy dog
the quiet green fir looms over the lazy dog
Send reinforcements; we're going to advance
Send 3 and sixpence; we're going to a dance
Time flies like an arrow
Dung flies like a  brick