//'s not necessary if $regex was built with qr.

Actually, if you leave out the slashes, the regex is put in string context, and the speed gain is lost.

$foo =~ '^string literal$';   # 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
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I've even seen people do bizarre things because they think qr//s are faster without slashes:

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

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