use strict;
use warnings;
use feature 'say';
use Data::Dump 'dd';
use Time::HiRes 'time';

my $N = 1000;
srand 123;
my @tokens = map { int rand 1e9 } 1 .. $N;

sub build_regex { qr/^.+123/m }
sub build_regex2 { qr/^.+\K123/m }

{                       # case 1
my $t = time;
my $count = 0;
for my $token ( @tokens )
{
    my $regex = build_regex( $token );
    /$regex/ && $count++ for @tokens;
}
say $count;
say time - $t;
}

{                       # case 2
my $t = time;
my $count = 0;
my $concat = join "\n", @tokens;
for my $token ( @tokens ) {
    my $regex = build_regex( $token );
    $count++ while $concat =~ /$regex/g
}
say time - $t;
}

{                       # case 3
my $t = time;
my $count = 0;
my $concat = join "\n", @tokens;
for my $token ( @tokens ) {
    my $regex = build_regex2( $token );
    $count++ while $concat =~ /$regex/g
}
say time - $t;
}

__END__

5000
0.384264945983887
1.7059121131897
0.13309907913208