sub1();
sub2();
sub3();
sub4();
sub5();
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Here is an example way to do what you want for educational purposes (Parallel::ForkManager is really the way to go in production code).

It is possible to loop through functions by taking references with the \& syntax:

sub sub1 { print 'sub1 ran' }
sub sub2 { print 'sub2 ran' }

my @refs = (\&sub1, \&sub2);
$_->() foreach @refs;
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Fork failure is indicated by an undef return value by the fork function. If you want to go on with processing in a non-parallel manner when fork fails, you can just check if it's defined. Full example:

use strict;
use warnings;
use POSIX ':sys_wait_h';

my @subs_to_run = (
    \&sub1,
    \&sub2,
    \&sub3,
    \&sub4,
    \&sub5,
);
foreach my $sub (@subs_to_run) {
    my $is_parent = fork;  # try to fork
    next if $is_parent;    # both child and fork failure will be false

    $sub->();  # we're in the child or fork failed, run the sub

    exit if defined $is_parent;  # exit only if actually forked
}

1 while waitpid(-1, WNOHANG) > 0;  # wait for all children
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It's more clear to me now, buy it will work only if the subs does not get any arguments, right? What if they does?
If the module you suggested can help us on solving this problem, I would love to see an example

Recover or die:

if ($pid = fork) {
    # parent
} else {
    die "cannot fork: $!" unless defined $pid;
    # child
}
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I prefer to use the fork method.

you mean thats what your homework said you should do