If it really is a string, substr and unpack could also be used. Here are three ways to do it:

use strict;
use warnings;

my $string = "PERLPERLPEBLPEBLPERL";
my $offset = 0;
my $step = 4;
my @array;

# unpack (could modify the pattern to use $step)
@array = unpack( '(A4)*', $string );

# substr, original string intact
while( $offset < length $string )
{
    push( @array, substr( $string, $offset, $step ) );
    $offset += $step;
}

# substr, original string destroyed
while( length $string )
{
    push( @array, substr( $string, 0, $step, '' ) );
}
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