$ cat lr.pl
#!/usr/bin/perl
use strict;
use warnings;

my @array = 1 .. 250_000;
for ( my $i = 250; $i <= @array; $i += 250 ) {
    my $next = by_groups_of( $i, \@array );
    while ( my $group = $next->() ) {
    }
}
sub by_groups_of {
    my ($by, $list) = @_;
    return sub { () } if ! $by || $by =~ /\D/ || ! @$list;

    my $pos = 0;
    my $done;

    return sub {
        return () if $done || $pos > $#$list;
        my $start = $pos;
        my $stop = $pos + $by - 1 > $#$list ? $#$list : $pos + $by - 1
+;
        $pos += $by;
        $done = 1 if $stop == $#$list;
        return sub { \@_ }->( @$list[ $start .. $stop ] );
    }
}

$ cat pn5.pl
#!/usr/bin/perl
use strict;
use warnings;

my @array = 1 .. 250_000;
for ( my $i = 250; $i <= @array; $i += 250 ) {
    my $rc_next = by_groups_of( $i, [ \(@array) ] );
    while ( my $ra_group = $rc_next->() ) {
        last unless @$ra_group;
    }
}
sub by_groups_of {
    my ($by, $ra_list) = @_;

    $by ||= 0;
    $ra_list ||= [];
    my $rc_return = sub { [] };

    if ($by && $by !~ /\D/ && @$ra_list)
    {
        my $pos = 0;
        my $done;

        $rc_return = sub {
            return [] if $done;
            my $start = $pos;
            my $stop = $pos + $by - 1 > $#{$ra_list} ? $#{$ra_list} : 
+$pos + $by - 1;
            #$pos += $by - 1;   # ?? bug ?? - changed to "$pos += $by;
+"
            $pos += $by;
            $done = 1 if $stop == $#{$ra_list};
            return [ @{$ra_list}[$start .. $stop] ];
        };
    }
    return $rc_return;
}

$ time ./lr.pl

real    1m39.558s
user    1m36.327s
sys     0m0.108s

$ time ./pn5.pl

real    4m48.570s
user    4m25.686s
sys     0m0.108s
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