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open(QTREES,$file1) or return print "foo";

open(QTREES,$file1) or (print "foo") && return;

$ touch file1; touch file2

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