I've mentioned this before, I'll explain it again.
The substitution s/^\s+// is simple and direct: replace a chunk of whitespace at the beginning of the string with nothing. The substitution s/\s+$// is simple and almost as direct: replace a chunk of whitespace followed by the end of the string with nothin. Sadly, it's not as "perfect" as the first one -- it matches each chunk of whitespace and examines it to see if it's at the end of the string or not. Oh well.
When you say s/^\s+|\s+$//g, perl's regex engine gets stupid. Real dumb. It tries BOTH branches of that regex at every location in the string. EVERY LOCATION! EVEN WHERE THERE AREN'T SPACES! See for yourself with perl -mre=debug -e '$_ = " a b c "; s/^\s+|\s+$//g' It's upsetting!