Your right. It was flawed. Another attempt.

#! perl -slw
use strict;

# Set up some test data.
my $text = 'The quick brown fox jumps over the lazy dog';
my @tests = map{
    my $t = $text;
    substr($t, rand( length $text ), 0 ) = $_;
    $t;
} map{
    substr 'vacation.msg', $_
} 0 ..6;

print "Before\n";
print for @tests;

#
s[(?:(?:(?:(?:(?:(?:v?a)?c)?a)?t)?i)?o)?n.msg][] for @tests;

print "\nAfter\n";
print for @tests;

__END__
P:\>junk
Before

The quick brown fox jumps ovvacation.msger the lazy dog
The quick brown fox jumps oacation.msgver the lazy dog
The quick brocation.msgwn fox jumps over the lazy dog
The quick brown fox jumpsation.msg over the lazy dog
The quick brown fox jumps over the lation.msgzy dog
Tion.msghe quick brown fox jumps over the lazy dog
The quick brown fox jumps over the lazy on.msgdog

After

The quick brown fox jumps over the lazy dog
The quick brown fox jumps over the lazy dog
The quick brown fox jumps over the lazy dog
The quick brown fox jumps over the lazy dog
The quick brown fox jumps over the lzy dog
The quick brown fox jumps over the lazy dog
The quick brown fox jumps over the lazy dog
[download]

This time, the apparent error is explainable.