Fair enough. Obviously, your reference to "It" wasn't specific enough. :-)

[Disclaimer: The following is my best guess. This isn't my area of expertise. I could be entirely wrong.]

In perlsub, under Localization of special variables, it says:

"If you localize a special variable, you'll be giving a new value to it, but its magic won't go away. That means that all side-effects related to this magic still work with the localized value."

Localizing %SIG and the $SIG{__WARN__} magic remains:

$ perl -wE 'warn 1; $SIG{__WARN__} = sub { say "WARN: $_[0]" }; warn 2
+; { local %SIG; warn 3 } warn 4'
1 at -e line 1.
WARN: 2 at -e line 1.

WARN: 3 at -e line 1.

WARN: 4 at -e line 1.
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However, localizing $SIG{__WARN__} and the magic is temporarily removed:

$ perl -wE 'warn 1; $SIG{__WARN__} = sub { say "WARN: $_[0]" }; warn 2
+; { local $SIG{__WARN__}; warn 3 } warn 4'
1 at -e line 1.
WARN: 2 at -e line 1.

3 at -e line 1.
WARN: 4 at -e line 1.
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