Because it's valid code, $two could be a boolean to be tested and as a side-effect $one is set.

try

 perl -e 'use strict; use warnings; my $one = 1; if( $one = 2 ) { prin
+t "True\n"; } else { print "False\n"; } '
Found = in conditional, should be == at -e line 1.
True
[download]

or

perl -we '$a=1; print  (($a = 2) ? "True" : "False");'
Found = in conditional, should be == at -e line 1.
[download]

cause now it makes less sense.

YMMV!

UPDATE

And personally I would be glad if this would cause a warning, too!