Dash (or minus or subtraction) followed by letter (or function name) causing syntax error in Perl -

i surprised when

sub f{1};  $answer = 1-f(1); 

gave me syntax error in perl when expecting perform subtraction. adding space made work okay again:

sub f{1};  $answer = 1- f(1); 

why cause syntax error in perl? there ambiguity? dash interpreted part of function name?

-f tests whether file plain file, not make sense perl not see - sign. why works expect, there no -fo test.

sub fo{1} $answer = 1-fo(1);   

(nor there +f test)

sub f{1} $answer = 1+f(1);