use Regexp::Assemble;
use Regexp::Exhaustive qw(exhaustive);
use List::Util qw(reduce);

my @keys = map { quotemeta } keys %hash;

my $key_re = Regexp::Assemble->new->add(@keys)->re;

for my $string (@strings)
{
    my $match =
      reduce { length($a) > length($b) ? $a : $b }
      exhaustive($string => qr/\b($key_re)\b/i);

    print "Found '$match' in '$string'\n"
      if defined $match;
}