use warnings;
use strict;
use Data::Dump;

my $input = <<'(END INPUT)';
abcdef
abcdefg
abcde
 bcdefgh
 bcd	
(END INPUT)

my @lines = split /\n/, $input;
dd \@lines;
my $out = make_rectangular( \@lines, 4, 6 );
dd $out;

sub make_rectangular {
	my ( $lines, $maxrows, $maxlength ) = @_;
	my @out;
	my $rowcount=1;
	for my $line (@$lines) {
		my $trimmed = substr $line, 0, $maxlength;
		push @out, sprintf "%-*s", $maxlength, $trimmed;
		last if ++$rowcount>$maxrows;
	}
	return \@out;
}

__END__

["abcdef", "abcdefg", "abcde", " bcdefgh", " bcd\t"]
["abcdef", "abcdef", "abcde ", " bcdef"]