my $oldhandler = $SIG{SOMETHING};
$SIG{SOMETHING} = sub {
 # do whatever 
 if ($oldhandler) {
    $oldhandler->();
 }
};
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1. this assumes that the other code does something like this as well, or that your handler is installed after the old one. See for instance my pretty simple CGI::HTMLError.

2. some kinds of signals (like SIGCHLD) are more complicated than this and may need special care.

I'd like to thank both of you for responding.

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