$foo =~ '^string literal$';   # Does not equal /\Q^string literal$/
$foo =~ $string;
$foo =~ subroutine();
$foo =~ bareword;             # Unless $INC{'strict.pm'}
$foo =~ $precompiled_regex;   # Is stringified first!

$foo =~ /$precompiled_regex/; # Faster

##</code><code>##

# Bad: (well, it works, but not the way you want it to)
$foo =~ $qr;
$foo =~ s//bar/;

# Good:
$foo =~ s/$qr/bar/;