Waiting for AnomalousMonks expert answer...

"ex" == "formerly"     "spurt" == "a drip under pressure"
"expert" == "ex" + "spurt"
"expert" == "formerly a drip under pressure"

I was thinking of something along the lines of johngg's extractive approach:
    @ra = $string =~ m{ [^Nn]+ }xmsg
I shied away from  [ACGT]+ because the presence of 'N' suggests the presence of other sequence characters (codon sequences? protein sequences? I'm not a bio-guy) than these. However, the problem with  [^Nn]+ is it assumes that the input sequences are correct: any junk other than 'N' or 'n' that happens to be present will also be extracted. Also, I share the confusion of others about what should happen to leading and trailing "NNN..." sub-sequences.