I need to do multiple substitutions in several large (1-10MB) files. I've been using this:

use strict;
use warnings;
use Tie::File;

foreach my $x (@ARGV) {
    tie my @f, 'Tie::File', $x or die "Could not tie $x: $!\n";
    for (@f) {
        s/^\|/\\N\|/;
        s/\|\s*$/\|\\N/;
        s/\|\s*\|/\|\\N\|/g;
        s/\|\.\s*\|/\|\\N\|/g;
        s/\|\s+/\|/g;
        s/\s+\|/\|/g;
        s/(\d{2}:\d{2}:\d{2})\.\d+/$1/g;
        s/(\d{5})-(?:\d{1,4}|\s+)/$1/;
    }
}
[download]

...but this is taking entirely too long, and using up too much CPU. How can I do this more efficiently?