Weird. I can reproduce OPs issue: 
$ cat /tmp/x
my $regex = '(2[0-4]|1?[0-9])?[0-9]|25[0-5]';
while (<>) {
    chomp;
    if ($_ =~ /^$regex$/) {
        print "$_ matched\n";
    } else {
        print "$_ did not match\n";
    }
}
$ perl /tmp/x
100
100 matched
200
200 matched
300
300 matched
^C

[download]
In fact adding any "|<something>" seems to trigger it, i.e. 
my $regex = '(2[0-4]|1?[0-9])?[0-9]|a';

[download]
gives the exact same result, and additionally matches anything starting with "a".

Aha. Looks like switching from 

my $regex = '(2[0-4]|1?[0-9])?[0-9]|25[0-5]';

[download]
to 
my $regex = qr/(2[0-4]|1?[0-9])?[0-9]|25[0-5]/;

[download]
seems to fix it. I don't immediately see why though.

In reply to Re^2: Is this a bug in perl regex engine or in my brain? by Crackers2
in thread Is this a bug in perl regex engine or in my brain? by nikmit

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