I never claimed significant speed improvement. What I had in mind is debugging thru the perl debugger. In that case the print statement becomes noise. I don't want anything to print, I even don't want to bother to go thru the statement that contains the print. Oddly, with use constant DEBUG => 0 I indeed step thru the statement that contains the print:  print "whatever\n" if DEBUG.
Worse I also step thru one line of constant.pm!

With sub DEBUG() { 0 } I still step thru the statement that contains the print. When debugging one goes from one nextstate opcode to the next. We see that consecutive nextstate statements are not fusionned as I expected them to be:

Both oneliners give me the same tree: perl -e ' sub DEBUG() { 0 } ; use O qw( Concise -exec); print "toto" if DEBUG; print "toto" if DEBUG'

perl -e ' use constant DEBUG =>0; use O qw( Concise -exec); print "tot
+o" if DEBUG; print "toto" if DEBUG'
1  <0> enter
2  <;> nextstate(main 1 -e:1) v
3  <;> nextstate(main 1 -e:1) v
4  <@> leave[t1] vKP/REFC
[download]

I don't understand how I get thru one line of constant.pm when debugging using constants!!!

I tested using perl 5.6.1

-- stefp