use warnings;
my %flows = ( '0' => 1, '00' => 2, '0$' => 3, '0$more' => 4, 'foo0$' => 5 );

foreach (keys %flows) {
    # loop through keys of %flows setting $_ to each key in turn
    if (!($_ =~ "^0\$")) {
        print "Orig $_ not matched\n";
    } else {
        print "Orig $_ matched\n";
    }

    if (!($_ =~ m"^0\$")) {
        print "RE   $_ not matched\n";
    } else {
        print "RE   $_ matched\n";
    }
    print $/;
}


__DATA__
Orig 0 matched
RE   0 not matched

Orig 00 not matched
RE   00 not matched

Orig 0$more not matched
RE   0$more matched

Orig 0$ not matched
RE   0$ matched

Orig foo0$ not matched
RE   foo0$ not matched