This is interesting! I see your point! I'm trying more tests now...Got it I hope!

Update: Ok, my wonky guess is that the pattern match returns a null string (not undef) from the '!~' which in an if statement is FALSE of course!

I think the index function technique may be faster if you're not using other pattern matching metacharacters. Run below to see what I think may explain the problem.

use strict;
my $exclude = 'middle_name pager zip';
my $lookfor='pager';
my $lookfor2='wonk';

# This would work without using pattern matching:
# Note lc function used to ignore case!
my $pos=index(lc($exclude),lc($lookfor),0);
if ($pos > -1) {print "Found $lookfor !\n";}
$pos=index(lc($exclude),lc($lookfor2),0);
if ($pos == -1) {print "Cannot find $lookfor2 !\n";}
print "\n";

my $look=($exclude =~ /$lookfor/gi);
my $look2=($exclude =~ /$lookfor2/gi);
my $notlook=($exclude !~ /$lookfor/gi);
my $notlook2=($exclude !~ /$lookfor2/gi);

print "'\$exclude =~ /$lookfor/gi' evaluates to '$look'\n";
print "'\$exclude =~ /$lookfor2/gi' evaluates to '$look2'\n";
print "'\$exclude !~ /$lookfor/gi' evaluates to '$notlook'\n";
print "'\$exclude !~ /$lookfor2/gi' evaluates to '$notlook2'\n";
__END__
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