As BrowserUk said, threads may be an easier choice. I noodled around with this before I saw his reply and came up with this adaptation of your script.

#!/usr/bin/perl

use warnings;
use strict;
use Data::Dumper;

use threads;
use threads::shared;

my %count : shared;
my $maxthreads = 3;

# there are way more lines than this but anyway...
for my $i ( 1 .. 500 ) {
    redo if ( scalar threads->list() >= $maxthreads );
    # this just represents that I've found a pair and want to run the 
+sub
    if ( $i % 50 == 0 ) {
        # run the sub
        my $thread = threads->new( \&inc, $i, 2 );
    }
}
while ( scalar threads->list() ) { sleep 1 }

# I need access to the hash that's updated in the sub
print Dumper \%count;

sub inc {
    my ( $param1, $param2 ) = @_;
    #long subroutine using passed params
    # pausing for effect!
    # this simulates that the sub takes a while to run
    sleep( 5 + ( rand 5 ) );
    
    # let you know something is happening
    print "$param1\n";

    # make a change to the hash
    $count{$param1} = $param1 * $param2;
    threads->self()->detach;
}
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