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

##</code><code>##

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
}