Since you are just a few months into perl, here is the canonical way to do this (Note - I changed the name and meaning of your variable for clarity):

my $pfaminput_filename ='D:/ARP/Downloads/Trembl/toxinpospf.txt'; 
       # Double fwd-slash is not necessary

open (my $pfaminput_filehandle , "<" , $pfaminput_filename ) 
         or die "Can't open '$pfaminput_filename' : Error is : $!";

while (my $pfam = <$pfaminput_filehandle>){
   print $pfam;
}

close $pfaminput_filehandle;
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Update: Corrected variable name inside error message.