You'd be wanting this I expect...

  sub flip { ${$_[0]} = !${$_[0]} };
[download]

The precedence of de-reference $ and array subscript [] mean that what you wrote doesn't work. You need those ${ ... } de-reference brackets for all de-references except the simplest.

Note also that you meant $_[0] rather than @_[0] didn't you ;-)

Interestingly

  sub flip { $$_[0] = !($$_[0]) }
[download]

Isn't a syntax error and doesn't produce any warnings but does something completely unintended.

  perl -w -e 'use strict; sub flip { $$_[0] = !($$_[0]) }; my $z=1; fl
+ip(\$z); print "$z\n"'
  1
[download]

I don't understand why this doesn't print use of unitialised variable or something like that...