Okay I am trying to figure out why 'one' works but 'two' just hangs the script. I read perlre and perlretut again to see if the answer would just jump out at me and it didn't.

Edit: I forgot to paste part of the code below. See my follow-up comment for the correct full program.

#!/usr/bin/perl

use v5.16;
use warnings;
use autodie qw( :all );
use utf8::all;
use File::Slurp qw( read_file );
use Regexp::Assemble;
use Benchmark qw( cmpthese :hireswallclock );

my %seen;
my %seen2;

my $fname   = 'dracula.txt';
my $content = read_file($fname);
$content =~ tr/!"#$%&'()*+,\-.\/:;<=>?@\[\\]^_`{|}~/ /;

my @patterns = read_file('sample_patterns');
chomp @patterns;
my $regex = join '|', map {quotemeta} @patterns;
$regex = qr/\b($regex)\b/ixms;


cmpthese(
    -5,
    {   
        'one' => sub {
            $seen{$1}++ while $content =~ /$regex/g;
        },
        'two' => sub {
            $seen2{$1}++ while $content =~ /$regex/;
        },
    }
);
[download]

The source text is Bram Stoker's Dracula a 836KB file with 16,248 lines. The sample_patterns file contains 4,000 patterns, one per line. The only difference between 'one' and 'two' is the g modifier on the regexp.