... with look-ahead ...

$+[0] also works here (note  $+[0] vice  $-[1] and no  /g modifier):

c:\@Work\Perl\monks>perl -wMstrict -le
"my $vowel = qr{ [AEIOUaeiou] }xms;
 ;;
 my $s = 'xyzzyBARxkcdFOOyz';
 ;;
 print 'offset of first vowel: ', $s =~ m{ (?= $vowel) }xms ? $+[0] : 
+'none';
 print 'offset of last vowel: ', $s =~ m{ .* (?= $vowel) }xms ? $+[0] 
+: 'none';
 print 'offset of first vowel: ', 'XYZ' =~ m{ (?= $vowel) }xms ? $+[0]
+ : 'none';
"
offset of first vowel: 6
offset of last vowel: 14
offset of first vowel: none
[download]

I like the look-ahead idea because it avoids a capture and so may be slightly faster.

To negate, one can use negative look-ahead (?! $vowel ).

I think there's a problem here:

c:\@Work\Perl\monks>perl -wMstrict -le
"my $vowel = qr{ [AEIOUaeiou] }xms;
 ;;
 my $s = 'aePDQioVWXua';
 ;;
 print 'offset of first non-vowel: ', $s =~ m{ (?! $vowel) }xmsg ? pos
+($s) : 'none';
 print 'offset of last non-vowel: ', $s =~ m{ .* (?! $vowel) }xmsg ? p
+os($s) : 'none';
 ;;
 $s = 'aei';
 print 'offset of first non-vowel: ', $s =~ m{ (?! $vowel) }xmsg ? pos
+($s) : 'none';
"
offset of first non-vowel: 2
offset of last non-vowel: 12
offset of first non-vowel: 3
[download]

The second and third cases give questionable results because there's always a place in a string where a negative look-ahead will succeed if it is true nowhere else: just beyond the end of the string. ($+[0] has the same problem in these cases as pos.)

Update: However, a positive look-ahead to a negated char class works:

c:\@Work\Perl\monks>perl -wMstrict -le
"my $non_vowel = qr{ [^AEIOUaeiou] }xms;
 ;;
 my $s = 'aePDQioVWXua';
 ;;
 print 'offset of first non-vowel: ', $s =~ m{ (?= $non_vowel) }xmsg ?
+ pos($s) : 'none';
 print 'offset of last non-vowel: ', $s =~ m{ .* (?= $non_vowel) }xmsg
+ ? pos($s) : 'none';
 ;;
 $s = 'aei';
 print 'offset of first non-vowel: ', $s =~ m{ (?= $non_vowel) }xmsg ?
+ pos($s) : 'none';
"
offset of first non-vowel: 2
offset of last non-vowel: 9
offset of first non-vowel: none
[download]

The positive assertion requires that something be present, so the overall match can fail. ($+[0] works as well as pos.)