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;
Update: Corrected variable name inside error message.
I hope life isn't a big joke, because I don't get it.
-SNL