You're running into the helpful inclination of Perl to freely interconvert strings and numbers in situations where it seems to make sense (and even sometimes where it doesn't). As | Similarly to what choroba said, what do you think  substr '123456', 0, 1 is?

c:\@Work\Perl\monks>perl -wMstrict -le
"use Data::Dump qw(dd);
 ;;
 my $string = pack('a*', '123456');
 dd $string;
 ;;
 print substr $string, 0, 1;
 printf qq{%02x \n}, substr $string, 0, 1;
 printf qq{%02x \n}, '1';
 ;;
 foreach my $c (unpack 'C*', $string) { printf '%02x ', $c; }
 print '';
 ;;
 $string .= qq{\x1c};
 dd $string;
 ;;
 foreach my $c (unpack 'C*', $string) { printf '%02x ', $c; }
"
123456

1
01
01
31 32 33 34 35 36
"123456\34"

31 32 33 34 35 36 1c
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Update: Finally noticed that choroba didn't actually say anything about substr, but rather about pack. Still, I think the question about substr is valid.