Question : Negate of existing String inside of RegEx

Hi,

I need to build a RegEx where the following String will be found:

PERFORM XYZ-SECTION
or
perform abc-section

I need to exclude this:

perform varying i1 from 1 by 1      
or
PERFORM VARYING I1 FROM 1 BY 1

All case-insensitive.

I found the examples above with the follwing RegEx:
$line=~ m/^.{6}\s+PERFORM\s+(\S+)($|\s|\./i

But with the given RegEx I found the loop's "Perform Varying" too. How build this RegEx to exclude the loops.
Using Perl as language for my RegEx.

Answer : Negate of existing String inside of RegEx

1:	$line=~ m/^.{6}\s+PERFORM\s+(?!varying)(\S+)($|\s|\./i