T'is easier with threads:

#! perl -slw
use strict;
use threads;

my $run = 0;
while( 1 ) {
    async {
        my $gna;
        for ( 'aaaaa' .. 'bbbbb' ) {
           $gna = uc;
        }
    }->detach;
    sleep 1;
    printf "\r%d", ++$run;
}
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