Using a /(foo|bar)/ regex on strings is slower than using a foreach loop doing the matching one after another.

Not in my benchmark:

use strict;
use warnings;
use Benchmark 'cmpthese';
undef $/;
my $str = <DATA>;

cmpthese(-2, {
  alternation => sub { my @m = $str =~ /(foo|bar)/g },
  map         => sub { my @m = map $str =~ /$_/g, qw(foo bar) },
  loop        => sub { my @m; push @m, $str =~ /$_/g for qw(foo bar) }
+,
});

cmpthese(-2, {
  alt_s  => sub { my $s2 = $str; $s2 =~ s/(foo|bar)//g },
  loop_s => sub { my $s2 = $str; $s2 =~ s/$_//g for qw(foo bar) }
});

__DATA__
This string contains foo and bar for fools and bards
and I pity the foo who bars the way
[download]

               Rate         map        loop alternation
map         14977/s          --         -6%        -25%
loop        15986/s          7%          --        -20%
alternation 20090/s         34%         26%          --
          Rate loop_s  alt_s
loop_s 17436/s     --   -37%
alt_s  27643/s    59%     --
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Update:
But I converted your example program to use Benchmark, and I do get better results with for:

use strict; 
use Benchmark 'cmpthese';

foreach my $regexcount (10) 
{ 
    foreach my $regexlength (5,20) { 
        my @items    = map{ createRandomTextWithLength($regexlength) }
+ (1..$regexcount); 
        my $regexstr = join('|',@items); 
        my $regex    = qr/(?:$regexstr)/; 

        foreach my $stringlength (1000,100000) { 
            print join "\n", "Stringlength: $stringlength",
                "Number of Regexes:$regexcount", "Length of each Regex
+:$regexlength\n"; 
            my $teststring = createRandomTextWithLength($stringlength)
+;
            cmpthese(-2, {
                alt => sub {
                    my $test=$teststring; 
                    $test =~ s/$regex/foobar/g; 
                },
                for => sub {
                    my $test=$teststring;
                    foreach my $oneregex (@items) {
                        $test =~ s/$oneregex/foobar/g;
                    }
                }
            });
        } 
    }
} 

sub createRandomTextWithLength($) 
{ 
    my($count) = (@_); 
    my $string; 
    for (1.. $count) { $string.=chr(ord('a')+rand(20)) } 
    return $string; 
}
[download]

Results:

Stringlength: 1000
Number of Regexes:10
Length of each Regex:5
      Rate  alt  for
alt 1296/s   -- -70%
for 4357/s 236%   --
Stringlength: 100000
Number of Regexes:10
Length of each Regex:5
      Rate   alt   for
alt 12.7/s    --  -97%
for  379/s 2877%    --
Stringlength: 1000
Number of Regexes:10
Length of each Regex:20
      Rate  alt  for
alt 1324/s   -- -68%
for 4144/s 213%   --
Stringlength: 100000
Number of Regexes:10
Length of each Regex:20
      Rate   alt   for
alt 12.7/s    --  -98%
for  676/s 5209%    --
[download]