More (although not as neat):
my $one = !$|;
my $one = cos; # Given that $_ == 0.
my $one = cos $|;
my $one = cos atan2 $|, cos;
my $one = sin atan2 sin, $|;
my $one = length $/; # Note: possibly 2 under Windows or DOS.
my $one = length $";
my $one = length $;;
my $one = length $^L;
my $one = $^T/$^T;
my $one = @ARGV >= 0;
my $one = grep //, ($_);
my $one = !undef;
# Note: the following may be Unix-only. Will not work if run as root,
# as root's UID and GID are (always?) zero.
my $one = $</$<; # User ID.
my $one = $>/$>; # Effective user ID.
my $one = $(/$(; # (Space-separated list of) group ID(s).
my $one = $)/$); # (Space-separated list of) effective group ID(s
+).
Ways to get 2:
my $two = length $:;
my $two = $^F; # Note: system-dependent.
