-e $F or return 0;   # File exists?
  -f $F or return 0;   # It's a plain file?
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You don't need both tests because if -f $F is true then -e $F has to be true.

$ mkdir test
$ mkdir test/file1.txt
$ touch test/file2.txt
$ perl -le'print "abcdefg"' > test/file3.txt
$ perl -le'print "\x90\x91\x92\x93\x94\x95\x96"' > test/file4.txt
$ perl -e'for my $file ( @ARGV ) { print "$file: "; if ( -e $file ) { 
+print "EXISTS " } if ( -f $file ) { print "PLAIN " } print "\n" }' te
+st/file*
test/file1.txt: EXISTS 
test/file2.txt: EXISTS PLAIN 
test/file3.txt: EXISTS PLAIN 
test/file4.txt: EXISTS PLAIN
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