perldoc -f grep

That faq is somewhat out of date, as recent perls have been optimized such that map and grep don't build return lists if they're not required. As the benchmarking code below shows (numbers generated with 5.6.1 on a 1.2G Athlon):

#!/usr/bin/perl
use Benchmark qw( timethese cmpthese );

my @args = ( 100_000,
             {
              void => sub {
                grep { $_ == 50 } 0..100; 1
              },
              scalar => sub {
                my $res = grep { $_ == 50 } 0..100
              },
              list => sub {
                my @res = grep { $_ == 50 } 0..100
              },
              a_foreach => sub {
                my $match;
                foreach( 0..100 ) { 
                  $match++ if $_ == 50
                }
                $match
              }
             } );

cmpthese( @args );

exit 0;

__END__
[download]

Benchmark: timing 100000 iterations of a_foreach, list, scalar, void...
 a_foreach:  4 wallclock secs ( 4.52 usr +  0.01 sys =  4.53 CPU) @ 22075.06/s (n=100000)
      list:  4 wallclock secs ( 3.35 usr +  0.00 sys =  3.35 CPU) @ 29850.75/s (n=100000)
    scalar:  3 wallclock secs ( 3.34 usr +  0.00 sys =  3.34 CPU) @ 29940.12/s (n=100000)
      void:  4 wallclock secs ( 3.29 usr +  0.00 sys =  3.29 CPU) @ 30395.14/s (n=100000)
             Rate a_foreach      list    scalar      void
a_foreach 22075/s        --      -26%      -26%      -27%
list      29851/s       35%        --       -0%       -2%
scalar    29940/s       36%        0%        --       -1%
void      30395/s       38%        2%        2%        --

There's practically no difference between grep in scalar and list context, while a foreach loop runs 26% slower.

Update: Oops, I'd said foreach was 35% slower. Changed the last paragraph above.

Update: Playing around, if you use grep EXPR, LIST rather than grep BLOCK LIST (i.e. grep $_ == 50, 0..100), that gap grows to 33% (since there's not as much overhead entering and leaving the block). If you switch comparisons from $_ == 50 to /50/ and stick with the BLOCK form, the foreach form is 45% faster.

a_foreach could be faster, as it is in perlfaq4.