foreach my $alpha ( @alphas ) {
  foreach my $beta ( @betas ) {
    if ( $alpha eq $beta ) {
       # do something
    }
  }
}

##</code><code>##

my %alpha;
@alpha{ @alphas } = undef; # suppress void context warnings

foreach my $beta ( @betas ) {
  if ( exists $alpha{ $beta } ) {
    # do something
  }
}