I linked to perlretut. Going there, I find:

The naive regexp

    ...
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doesn't work; it may match a TGA , but there is no guarantee that the match is aligned with codon boundaries, e.g., the substring GTT GAA gives a match. A better solution is

    while ($dna =~ /(\w\w\w)*?TGA/g) { # note the minimal *?
    print "Got a TGA stop codon at position ", pos $dna, "\n";
    }
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which prints

    Got a TGA stop codon at position 18
    Got a TGA stop codon at position 23
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Position 18 is good, but position 23 is bogus. What happened?

Maybe it was too obvious for me, but a Codon is a nucleotide triplet.