I'm not so sure whether I grasp the correct connotation of 'match' in that context. But, consider using:

use strict;
use warnings;

my $str = 'abc' x 10;

if (length $str == 30) {
    print "\$str has length of 30\n";
}
[download]

print $str =~ /(.{30})/;
[download]

Former one succeeds if $str has a length of 30 characters. Latter one yields back the first 30 characters extracted from $str using a regular expression.