use strict;

my @a = (1..20);
my @b = (2,3,13,14,17);

@a = do{my %i; undef @i{@a}; delete @i{@b}; sort {$a<=>$b} keys %i};

##</code><code>##


use strict;

#Lets say we have two arrays @a and @b
#and we want to, as in your posting, remove the elements in @b
#from those in @a

my @a = (1..20);
my @b = (5,8,13,16);

#we in essence re-write the contents of @a with our map statement
#that allows us to check the values for those we want to remove.
@a = map{
    my $i = $_; 
    (grep /^$i$/, @b) ? () : $i ;
    }@a;