There have been concise explanations for the
behavior you found.

I'd use a core module (List::Util) for that:

 use List::Util qw(first);

 my @arr=('cool','guy','here');
 my $str1="I am cool";

 my $hit = first{ $str1 =~ /\Q$_/ } @arr;
 print "Matched $hit";
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Regards

mwa