open my $FH, "<", $in or (print "cannot open file $in\n" and exit 3);

##</code><code>##

$ perl -e 'open my $FH, "<", "xml.pl" and print "Opened!" or print "cannot open file\n" and print 3'
Opened!
$ perl -e 'open my $FH, "<", "xml2.pl" and print "Opened!" or print "cannot open file\n" and print 3'
cannot open file
3