Hi,

Here is a brute force try....

sub common {
    my ($a,$b)=@_;
    my ($long,$short);
    if (length ($a) <= length ($b)){
        ($long,$short) = ($b,$a);
    }
    else {
        ($long,$short) = ($a,$b);
    }
    my $i=0;
    my $common="";
    while ($i<length($short)){
        my $str = substr($short,$i,1);
        my $t=$i;
        $i++;
        next unless ($long=~/$str/);
        while ($long=~/$str/) {
            $t++;
            last unless (my $uni = substr($short,$t,1));
            last unless ($long=~m/$str$uni/);
            $str.=$uni;

            }
        $common = $str if (length($common)<length($str));

    } 
    return $common;
}
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