grep is usually used to sift through an array or list. You're passing it a scalar variable, not a list. That's one problem. What do you think $mk contains?

grep in scalar context will return a count of how many matches it found in the list passed to it. So it would seem that whatever it is that $mk contains, it does NOT contain anything that matches with the expression, "/^\Q$folder\E$/". Since it doesn't match (not even once), the number of hits is zero, and that's what gets passed back from grep to $folder_present. grep isn't failing, as your post's title suggests. It's doing what it's designed to do.

What are you trying to accomplish? I don't mean what do you want this little snippet to do. I mean what is the bigger picture here? What is your goal?

Based off your very limited description, I'm surmising that $mk contains a return delimited list of folders and you're trying to determine if $folder is present.

As davido already pointed out, grep is not what you need for this because that is meant only for array operations. If you're just working on a scalar, you can do the test directly. Secondly, based off your regex, I suspect your problem is you're not using the 'm' modifier which would make it so ^ and $ matched the beginning and end of any line within the text, not just the start and end of the string.

Here's an example that does match like you want

my $mk = <<'END_LIST';
/foo/bar
/foo/baz
/eep/foo/bar
/epp/foo/baz
/biz
/baz/foo/bar
END_LIST

my $folder = '/foo/baz';

my $folder_present = $mk =~ /^\Q$folder\E$/im;

print "$folder_present"; # Prints 1
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- Miller

Does anyone have any idea why the below if condition is failing for the given input?

$folder:FileMuxInternaldefs


$mk:./AACBaseFileLib/common.mk:  $(PROJECT_ROOT)/../FilemuxInternalDef
+s/inc  \

               if ($folder =~ /$mk/i) #this match failing for the give
+n input
                {
                      print "\nIN\n";
                                  if ($folder !~ /$mk/m)
                                 {
                                    print "CASE-SENSITIVE:$mk\n";
                        
                                 }
                }
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Probably for the same reason when you asked here, FOR LOOP not entered.

Your code does not actually run because of this nonsense

$folder:FileMuxInternaldefs


$mk:./AACBaseFileLib/common.mk:  $(PROJECT_ROOT)/../FilemuxInternalDef
+s/inc  \
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