Your IRS_while(), does not return the same information as the other two subroutines. Push returns the size of the array after update. Assignment returns the assigned value. Is it possible that you are getting bogus results due to that?

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

my $count = -100;
cmpthese($count, {
    'Split' => sub {
        my $document;
        open(FILE, 'removed.xml') or die "Error [$!]\n"; 
        while (<FILE>) { $document .= $_ }
        my @lines = split('\|',$document);
    return @lines;
    },
    'IRS_while' => sub {
        local $/ = '|';
        my @lines;
        open(FILE, 'removed.xml') or die "Error [$!]\n"; 
        while (<FILE>) {
            chomp;
            push @lines, $_;            
        }
    return @lines;
    },
    'IRS_map' => sub {
        local $/ = '|';
        open(FILE, 'removed.xml') or die "Error [$!]\n"; 
        my @lines = map {chomp; $_} (<FILE>);
    return @lines;
    },
});
[download]

Results:

             Rate   IRS_map     Split IRS_while
IRS_map    9804/s        --      -24%      -27%
Split     12876/s       31%        --       -4%
IRS_while 13399/s       37%        4%        --
[download]

and

This is perl 5, version 12, subversion 3 (v5.12.3) built for MSWin32-x
+86-multi-thread
[download]

Benchmark newbie, so I am certain that I will be corrected if I have also misapplied the tool :-)

Update: To tie into a comment by 2teez, my versions of this seem to be spending a large amount of time in open (32% of total time) and readline (18% of total time) -- a whopping 50% of the total time, with those two items being the top two runners when sorted by exclusive time under Devel::NYTProf.