use strict;
use warnings;

sub head {
   my ($cmd, $count) = (shift, shift || 10);
   my @output;

   open my $fh, "$cmd |" or die "error: '$cmd' failed";
   while (<$fh>) { push @output, $_; last if $. == $count; }
   close $fh;

   return @output;
}

my @foo = head('cat very_large_file', 4);

print @foo;