my $point = 15;
if ($str =~ /^(.{15})/ && $1 =~ /(regex)$/) {
    print "stuff before pos 15: $1\n";
}

##</code><code>##

my $point = 15;
my $start = 10;
if (substr($str, $start, $point - $start) =~ /(regex)$/) {
    print "stuff before pos: $1\n";
}

##</code><code>##

pos $str = 15;
print "matched\n" if $str =~ /\G(?<=regex)/g;