First of all, in negative look behind, variable length is not allowed, which you have used (.*).

If i understood your question correctly, this will work.

my $string = qq{oCMenu.makeMenu('m40','','Files','','',100,20) oCMenu.
+makeMenu('m41','m40','Periodic Reports','index.cfm?openaction=file_ar
+chive.view&CONTENT_ID=BF764E84-ED11-EC57-40672E520082E823','',125,20)
+ oCMenu.makeMenu('m53','m40','Reference Documents','index.cfm?openact
+ion=file_archive.view&CONTENT_ID=BF76E870-E7CC-515F-D4D5C3D4A210BB9A'
+,'',125,20)};

my ($match) = $string =~ /Periodic Reports','([^']*)/;  
print "$match\n";
#$string =~ /Periodic Reports','(?<!.*'$)/;  #not matches
[download]

Also take a look at perlre