I am unable to duplicate your problem with my perl v5.16.3.

I can duplicate the problem with both Strawberry 5.14.4.1 and ActiveState 5.8.9, both under Win32.

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=('0','M','0','A','0','I','0','L');
 printf $parr[floor( $PC1[$_]/8 )] for (0..63);
"
perl version 5.014004
Use of uninitialized value in printf at -e line 1.
L0I0A0M0L0I0A0M0L0I0A0M0L0I0A0ML0I0A0M0L0I0A0M0L0I0A0M0L0I0A0M0
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As in the OPed code, the problem arises from the element of 64 in the  @PC1 array producing an index of 8 into the  @parr array, the highest valid index of which is 7.