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$foo =~ '^string literal$';   # Does not equal /\Q^string literal$/
$foo =~ $string;
...
$foo =~ $precompiled_regex;   # Is stringified first!

$foo =~ /$precompiled_regex/; # Faster

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

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