$ perl -MTime::Piece -le 'print prototype("CORE::localtime"); print prototype("Time::Piece::localtime")//"undef"'
;$
undef

$ perl -MO=Deparse -e 'my $x = localtime; my $y = localtime - $x; print "$y\n";  '
Warning: Use of "localtime" without parentheses is ambiguous at -e line 1.
my $x = localtime;
my $y = localtime -$x;
print "$y\n";
-e syntax OK

$ perl -MO=Deparse -e 'use Time::Piece; my $x = localtime; my $y = localtime() - $x; print "$y\n";  '
use Time::Piece;
my $x = main::localtime();
my $y = main::localtime() - $x;
print "$y\n";

$ perl -MO=Deparse -e 'use Time::Piece; my $x = localtime; my $y = (localtime) - $x; print "$y\n";  '
use Time::Piece;
my $x = main::localtime();
my $y = main::localtime() - $x;
print "$y\n";

$ perl -MO=Deparse -e 'use Time::Piece; my $x = localtime; my $y = localtime - $x; print "$y\n";  '
use Time::Piece;
my $x = main::localtime();
my $y = main::localtime(-$x);
print "$y\n";