Hello FeistyLemur, and welcome to the Monastery!

See the section “Global matching” in perlretut#Using-regular-expressions-in-Perl, and in particular note the following (emphasis added.):

In scalar context, successive invocations against a string will have //g jump from match to match, keeping track of position in the string as it goes along....
A failed match or changing the target string resets the position.... The current position in the string is associated with the string, not the regexp.

Hope that helps,

Yes, I understand now why I was seeing the behavior in question. Things are working as I intended in my script. I just wanted to understand what I was missing about the match operator. I didn't realize the position in the string was associated with the string and not the regexp.

I got the answer I was looking for and understand the match operator better now, thanks.

... the negative match ...

It's important to understand that the negation is made on the result of the  /eth1\.\d{4}\@/gm match against the  $ethernet string. The match itself is actually successful! It's as if the
    if ($ethernet !~ /eth1\.\d{4}\@/gm) { ... }
statement had been written as
    if ( ! ($ethernet =~ /eth1\.\d{4}\@/gm)) { ... }
(note =~ vice !~). In fact, that's pretty much the way Perl sees the code:

c:\@Work\Perl\monks>perl -wMstrict -MO=Deparse,-p -le
"my $ethernet=`ip addr`;
 if ($ethernet!~/eth1\.\d{4}\@/gm){
     print qq{No vlans exist on device.\n};
     exit;
 }
"
BEGIN { $^W = 1; }
BEGIN { $/ = "\n"; $\ = "\n"; }
use strict 'refs';
(my $ethernet = `ip addr`);
unless (($ethernet =~ /eth1\.\d{4}\@/gm)) {
    print("No vlans exist on device.\n");
    exit;
}
-e syntax OK
[download]

I had assumed /g was pointless on the if ...

Given what you were trying to do, it was pointless, but it was not without effect!