When you match a regex against a string, you're essentially asking the question "is there any position in the string at which the pattern matches?" The regex engine (RE) is looking for any match, and normally will be satisfied by the first match (it's possible to force the RE to go on looking). The rule is "the leftmost, longest match", and "leftmost" trumps "longest."

c:\@Work\Perl\monks>perl -wMstrict -le
"for my $f ('xyz.pl', '.pl', 'abc.x', 'abc') {
   printf qq{%10s matches at position $-[0] \n}, qq{'$f'}
     if $f =~ m{ (?! [.]pl \z) }xms;
   }
"
  'xyz.pl' matches at position 0
     '.pl' matches at position 1
   'abc.x' matches at position 0
     'abc' matches at position 0
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In most of these examples, a match can be found at the very start of the string, the first position "not followed by .pl and the end of the string". Even in the case of  '.pl' a match can be found, even though the RE has to go all the way to the second character position to find it.