Hmm. The OPs spec was to match a WORD that starts with N and doesn't end with 00. All of the solutions I've seen posted so far work on a PHRASE (to be fair, including a phrase that consists of one word...) Fine if there is only going to be a single word to match against, but to be more general, use something like:

/\bN(?!\w*00\b)\w*\b/
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as in:

my $string = q/N553342 N455673 N55788 N44200 NZ31200 NZ3120A/;
my @matches = $string =~ /\bN(?!\w*00\b)\w*\b/g;
print join "\n", @matches;
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