Are we making this too complicated? If all you want is to create a character string of a given length, perhaps what you need is just the "x" command applied to a simple one-byte ascii character.

#!/usr/bin/perl -w
use strict;
 
my $len = 0;
if (@ARGV >= 1) {
  $len = $ARGV[0];
}
$len > 0 || die "Usage: genlen.pl number\n";
print "a" x $len;
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