Not sure if this is more elegant than the OPed solution or than JavaFan's, but...

>perl -wMstrict -le
"my $be24s = qq{\x01\x02\x03\x04\x05\x06\x07\x08\x09};
 printf qq{%08lx  }, $_ for unpack_be24($be24s);
 sub unpack_be24 {
   my ($s) = @_;
   return map unpack('N', qq{\x00$_}), unpack '(a3)*', $s;
   }
"
00010203  00040506  00070809
[download]

Again, to make this little-endian, append the null byte and use  'V' for the unpack template.

Note: Of course, this doesn't actually read a 24-bit integer as requested in the OP, but I figure the reading part should be fairly straightforward.

Update: A slightly different approach, with one less map and an extra pack:

>perl -wMstrict -le
"my $be24s = qq{\x11\x12\x13\x24\x25\x26\x37\x38\x39};
 my $le24s = reverse $be24s;
 printf qq{%08lx  }, $_ for unpack_be24($be24s);
 print '';
 printf qq{%08lx  }, $_ for unpack_le24($le24s);
 sub unpack_be24 {
   my ($s) = @_;
   return unpack 'N*', pack '(xa3)*', unpack '(a3)*', $s;
   }
 sub unpack_le24 {
   my ($s) = @_;
   return unpack 'V*', pack '(a3x)*', unpack '(a3)*', $s;
   }
"
00111213  00242526  00373839
00373839  00242526  00111213
[download]