You want a hash not an array for @saw.

#!/usr/bin/perl -w

use strict;

my @emails = qw/mgr@acme.net
ipadm@acme.net
abuse@acme.net
mgr@acme.net
ipadm@acme.net
abuse@acme.net
    /;

my %saw;
@emails = grep(!$saw{$_}++, @emails);

foreach (keys %saw) {
    print "$_\n";
}
[download]

grep

grep> cd pub 
grep> more beer