Also look at the explanation in the following snippet, from previous posts

#!/usr/bin/perl
#The difference between qr// and the /o modifier as I understand it is
+ 
#that with /o you can never change the pattern, whereas with qr// you 
#can compile the pattern to be used as part of a larger regex. 

@regexes = ();
for $pattern (@patterns) {
push @regexes, qr/$pattern/;
}
for $item (@data) {
for $re (@regexes) {
if ($item =~ /$re/) { print "Matches!\n"; }
}
}

#/o and qr// provide different services to the programmer:
#/o says 'Take this pattern and create a compiled regex that will not 
+change 
#for the life of the program'
#qr// says 'Take this pattern and return a special object that I can a
+ssign 
#to a variable, interpolate into another pattern, pass to a subroutine
+, 
#etc, etc, and as a side-effect creates a compiled form that I can use
+ in 
#pattern matches directly.'
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