List installed packages which are not depended by other installed packages. In other words, list all required packages.

not_depended_packages.pl

#!/usr/bin/perl

use strict;

&main();

sub main {
    open( my $ph, 'dpkg --status |' ) or die;
    my %package;
    my %depended;
    my $p;
    while( <$ph> ){
	s/\s+\Z//;
	if( s/\APackage:\s+// ){
	    $p = $_;
	}
	elsif( m/\AStatus: install/ ){
	    $package{$p}++;
	}
	elsif( s/\ADepends:\s+// ){
	    for my $x ( split( /,\s+/, $_ ) ){
		$x =~ s/\s+\(.*?\)\Z//;
		$x =~ s/:.*?\Z//;
		$depended{$x}++;
	    }
	}
    }
    for my $p ( sort keys %package ){
	next if $depended{$p};
	print $p, "\n";
    }
}