This is a small example of a "real-time" subroutine:

# calls this sub each 3 seconds...

local $SIG{ALRM} = sub {
        print "alarm ";
        alarm 3;
};

$| = 1;    # no output buffer
alarm 3;   # start subroutine timer

# main program

for (1..100) {
    print "running ";
    sleep 1;
}
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