use strict;
use warnings;

my @vars = ('and gugus for','gugus and', 'gurit for','granit');

print ("any string 'gu' followed by 'gu'\n");
foreach (@vars) {
    if ($_ =~ /gu(?=gu)/) { print ("found <$_>\n"); }
    else { print ("no match for <$_>\n"); }
}

print ("\nany string 'gu' not followed by 'gu'\n");
foreach (@vars) {
    if ($_ =~ /gu(?!gu)/) { print ("found <$_>\n"); }
    else { print ("no match for <$_>\n"); }
}