Further to lidden's reply:
Perl would have given you a clue to what was going on had you allowed it to by enabling warnings.

c:\@Work\Perl\monks>perl -wMstrict -le
"my @arr = ('jack', 80, 'martin', 3, 'allan', 'george');
 ;;
 my @sort = sort { $a <=> $b } @arr;
 print qq{@sort};
"
Argument "jack" isn't numeric in sort at -e line 1.
Argument "martin" isn't numeric in sort at -e line 1.
Argument "allan" isn't numeric in sort at -e line 1.
Argument "george" isn't numeric in sort at -e line 1.
jack martin allan george 3 80
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See also Lexicographical order for more info on the  cmp sort.

Update: See also Scalar values in perldata. Also, consider the following:

c:\@Work\Perl\monks>perl -wMstrict -le
"my @ra = qw(foo 2bar 123baz quux999 x999x);
 for my $s ('', @ra) {
   no warnings 'numeric';
   print qq{'$s' == }, 0+$s;
   }
"
'' == 0
'foo' == 0
'2bar' == 2
'123baz' == 123
'quux999' == 0
'x999x' == 0
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