#!/usr/bin/perl

use strict;
use warnings;

my $MAX = 100;

my $re = qr /^(1+)(??{"(?:$1){" . (length ($1) - 1) . "}" })$/;

for (1 .. $MAX)
    {printf "%3d is a square\n" => $_ if (1 x $_) =~ /$re/}
do  {printf "%3d is a square\n" => $_ if (1 x $_) =~ /$re/} for 1 .. $MAX;
map {printf "%3d is a square\n" => $_ if (1 x $_) =~ /$re/}     1 .. $MAX;

##</code><code>##

  1 is a square
  4 is a square
  9 is a square
 16 is a square
 25 is a square
 36 is a square
 49 is a square
 64 is a square
 81 is a square
100 is a square
  1 is a square
  4 is a square
  9 is a square
 16 is a square
 25 is a square
 36 is a square
 49 is a square
 64 is a square
 81 is a square
100 is a square
  1 is a square
  4 is a square
  9 is a square
 16 is a square
 25 is a square
 36 is a square
 49 is a square
Out of memory!

##</code><code>##

  1 is a square
  4 is a square
  9 is a square
 16 is a square
 25 is a square
 36 is a square
 49 is a square
 64 is a square
Out of memory!