So as I understand it, you want to get the string printed by the subroutine into a variable in the calling namespace. If this is what you want try something like this:

use IO::Pipe;
use strict;

my $pipe = IO::Pipe->new();


if(my $pid = fork()){
    waitpid($pid, 0);
    $pipe->reader();
    
    my $str = <$pipe>;

    print "\$str == '$str'\n";
}else{
    $pipe->writer();
    
    &print_to_pipe($pipe);
}


sub print_to_pipe
{
    my $fh = shift;
    print $fh "Hello World\n";
}
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See IO::Pipe.