In addition to the comments above, it is usually helpful to use diagnostics; (see perldoc diagnostics) when you receive a strange warning. For example,

$ perl -w -e 'close FILEHANDLE; print <FILEHANDLE>'
readline() on closed filehandle FILEHANDLE at -e line 1.
$ perl -w -Mdiagnostics -e 'close FILEHANDLE; print <FILEHANDLE>'
readline() on closed filehandle FILEHANDLE at -e line 1 (#1)
    (W closed) The filehandle you're reading from got itself closed so
+metime
    before now.  Check your control flow.
    
$ perl -w -Mdiagnostics -e 'my $s="text"; print <$s>'
readline() on unopened filehandle at -e line 1 (#1)
    (W unopened) An I/O operation was attempted on a filehandle that w
+as
    never initialized.  You need to do an open(), a sysopen(), or a so
+cket()
    call, or call a constructor from the FileHandle package.
    
$
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(see perldoc perlrun if you want to know about -M and -e arguments)

EDIT: perldoc links