use strict;
use warnings;

my @array = ( 0 .. 50 );

for ( my $idx = 0; $idx <= $#array; $idx += 20 ) {
    my $top = $idx + 19;
    $top = ( $top <= $#array ) ? $top : $#array;
    print "Processing elements $idx through $top.\n";
    foreach my $element ( @array[ $idx .. $top ] ) {
        # Process each $element here.  ...for example...
        print $element, " ";
    }
    print "\n\tDone with $idx through $top.\n\n";
}