Your math is unnecessarily complicated.

One could further simplify the math by taking advantage of the fact that the  [ ] subscript operator does its own 'floor' operation by just throwing away the fractional part of the (inherently) double number:

c:\@Work\Perl>perl -wMstrict -le
"use POSIX qw(floor);
 ;;
 print 'perl version ', $];
 ;;
 my @PC1=(58,50,42,34,26,18,10,2,
          60,52,44,36,28,20,12,4,
          62,54,46,38,30,22,14,6,
          64,56,48,40,32,24,16,8,
          57,49,41,33,25,17, 9,1,
          59,51,43,35,27,19,11,3,
          61,53,45,37,29,21,13,5,
          63,55,47,39,31,23,15,7);
 ;;
 my @parr = qw(0 M 0 A 0 I 0 L);
 printf $parr[floor( $PC1[$_]/8 )] for (0..63);
 print '';
 printf $parr[ $_ / 8 ] for @PC1;
"
perl version 5.014004
Use of uninitialized value in printf at -e line 1.
L0I0A0M0L0I0A0M0L0I0A0M0L0I0A0ML0I0A0M0L0I0A0M0L0I0A0M0L0I0A0M0
Use of uninitialized value in printf at -e line 1.
L0I0A0M0L0I0A0M0L0I0A0M0L0I0A0ML0I0A0M0L0I0A0M0L0I0A0M0L0I0A0M0
[download]

(Of course, this doesn't address the root problem of generating an index that's out of range!)