use strict;

my @a=qw(a b c d);
my @b=qw(b a e f);

# this does exactly what your pair of 'for' loops 
# did...
my @m = map {
    my $x = $_;
    grep { /^$x$/ } @b;      
} @a;
 
print @m;
print "\n";

##</code><code>##

ab

##</code><code>##

use strict;

my @a=qw(a b c d);
my @b=qw(b a e f);

my %h;
my @m = map {$h{$_}++ ? $_ : ()} (@a, @b);
 
print @m;
print "\n";

##</code><code>##

$"=q;grep;;$,=q"grep";for(`find . -name ".saves*~"`){s;$/;;;/(.*-(\d+)-.*)$/;
$_=["ps -e -o pid | "," $2 | "," -v "," "];`@$_`?{print"+ $1"}:{print"- $1"}&&`rm $1`;
print$\;}