Deparsing the code shows what Perl sees (in tmp.pl):

#!/usr/bin/perl -w
use strict;

sub ten { 10 }

print "9 .. 10 - 1 with constant\n";
for my $i (9 .. ten - 1) {
  print $i, "\n";
}
print "-> unexpected\n\n";
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gives when deparsed:

corion@outerlimits:~/Projekte$ perl -MO=Deparse tmp.pl 
BEGIN { $^W = 1; }
use strict;
sub ten {
    10;
}
print "9 .. 10 - 1 with constant\n";
foreach my $i (9 .. ten(-1)) {
    print $i, "\n";
}
print "-> unexpected\n\n";
tmp.pl syntax OK
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Since you don't have a prototype on your constant, the -1 gets interpreted as an argument to your subroutine call ten. The fix is to use a prototype, or to use the constant module, which does the same:

sub ten() { 10 };
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... or ...

use constant ten => 10;
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