$MAX=1e7; 
cmpthese -1, {
    a => q[
        my $c = 0; 
        $_%2 and ++$c for 0 .. $MAX; 
        print "a:$c" 
    ], 
    b => q[
        my $c = 0; 
        $_ =~ m[[02468]$] or ++$c for 0 .. $MAX; 
        print "b:$c"
    ] 
};;
a:5000000
a:5000000
b:5000000
b:5000000
  s/iter    b    a
b   3.50   -- -64%
a   1.27 177%   --

$MAX=1e8; 
cmpthese -1, { 
    a => q[my $c = 0; $_%2 and ++$c for 0 .. $MAX; print "a:$c" ], 
    b => q[ my $c = 0; $_=~ m[[02468]$] or ++$c for 0 .. $MAX; print "b:$c" ] 
};;
a:50000000
a:50000000
b:50000000
b:50000000
  s/iter    b    a
b   34.3   -- -64%
a   12.3 179%   --