$ touch file1; touch file2

$ perl blenge.pl

$ rm file2; perl blenge.pl
Error opening file2: No such file or directory

$ rm file1; perl blenge.pl
Error opening file1: No such file or directory

$ cat blenge.pl 
#!/usr/bin/perl
use strict;

doit();

sub doit {
    open QTREES, 'file1' or return print STDERR "Error opening file1: $!";
    open RTREES, 'file2' or (print STDERR "Error opening file2: $!") && return;
}