Methinks your logic is a little off... if I understand you correctly, you want to test to see if the file exists (with either a '1' or not in the filename. And if so, open it and do whatever. Otherwise, move on (or give an alert or something). The '-f' expression tests to see if the file is a regular file (on top of it existing), but that's not really what's hanging you up, I bet.

my $txtfile = $prefix . ".txt";
my $txtfile1 = $prefix . "1.txt";
if ( -f $txtfile ) {
 # do stuff
} elsif ( -f $txtfile1 ) {
 # do stuff
} else {
 # no file was found
}
[download]

You may want to even share whatever 'stuff' you have to do:

if ( -f $txtfile || -f $txtfile1 ) {
 if ( -f $txtfile ) { open(FILE, "$txtfile"); }
 else { open(FILE, "$txtfile1"); }
 # do stuff with FILE
 close (FILE);
} else {
 # no file was found
}
[download]

Disclaimer: Code is untested and there are probably better ways to streamline the code - aiming for readability.

Jason