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The given ... when construct does a smart match ~~ on the expression after the when EXCEPT in the following cases:

a subroutine or method call
a regular expression match, i.e. /REGEX/ or $foo =~ /REGEX/ , or a negated regular expression match (!/REGEX/ or $foo !~ /REGEX/ ).
a comparison such as $_ < 10 or $x eq "abc" (or of course $_ ~~ $c )
defined(...), exists(...), or eof(...)
a negated expression !(...) or not (...) , or a logical exclusive-or (...) xor (...)
a filetest operator, with the exception of -s , -M , -A , and -C , that return numerical values, not boolean ones.
the .. and ... flip-flop operators.

Regex-replacements are not in that list, but a negated expression is, therefore your !! trick worked.

CountZero

A program should be light and agile, its subroutines connected like a string of pearls. The spirit and intent of the program should be retained throughout. There should be neither too little or too much, neither needless loops nor useless variables, neither lack of structure nor overwhelming rigidity." - The Tao of Programming, 4.1 - Geoffrey James

In reply to Re: "when" and replacements by CountZero
in thread "when" and replacements by John M. Dlugosz

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