$ perl -Mstrict -Mwarnings -E '
    my %default_signal_for = %SIG;

    say "*** Starting values ***";
    say "SIG{USR1}: ", $SIG{USR1} // "<undef>";
    say "DEF{USR1}: ", $default_signal_for{USR1} // "<undef>";

    $SIG{USR1} = sub { 1 };

    say "*** After setting USR1 ***";
    say "SIG{USR1}: ", $SIG{USR1} // "<undef>";
    say "DEF{USR1}: ", $default_signal_for{USR1} // "<undef>";

    $SIG{USR1} = $default_signal_for{USR1};

    say "*** Back to default values ***";
    say "SIG{USR1}: ", $SIG{USR1} // "<undef>";
    say "DEF{USR1}: ", $default_signal_for{USR1} // "<undef>";
'
*** Starting values ***
SIG{USR1}: <undef>
DEF{USR1}: <undef>
*** After setting USR1 ***
SIG{USR1}: CODE(0x7fa4008401b0)
DEF{USR1}: <undef>
*** Back to default values ***
SIG{USR1}: <undef>
DEF{USR1}: <undef>