Your question makes no sense. In the code you show there is no point at which you are explicitly returning data from any of your subs and at no point is implicitly returned data from any sub used. Maybe you meant something like:

use warnings;
use strict;

print addIronman(addAm(addI("")));

sub addI {
    my ($string) = @_;
    $string . "i";
}

sub addAm {
    my ($string) = @_;
    return $string . " am";
}

sub addIronman {
    my ($string) = @_;
    return "$string ironman";
}
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Note that addI returns its result implicitly because the result is the last value evaluated before the implicit return at the end of the sub. The other subs return explicit values.