That depends on what you mean by play nicely. This will restore the previous signal handlers after leaving your function.

sub do_something {
    local $SIG{__WARN__} = sub { ... };
    # your stuff goes here
    ...
}   # previous version restored
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If you want to have your handler called and then the original then you will need to do a bit more work

my $old_warn = $SIG{__WARN__};
$SIG{__WARN__} = sub {
    # your stuff
    $old_warn->(@_);
}
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