This is why ‘use warnings' is important and ‘use diagnostics’ is helpful

gistfile1.txt

$ perl -E'open my $fh, "test"; print {$fh} "butts"' 
$ perl -Mwarnings -E'open my $fh, "test"; print {$fh} "butts"'
Filehandle $fh opened only for input at -e line 1.
$ perl -Mwarnings -Mdiagnostics -E'open my $fh, "test"; print {$fh} "butts"'
Filehandle $fh opened only for input at -e line 1 (#1)
    (W io) You tried to write on a read-only filehandle.  If you intended
    it to be a read-write filehandle, you needed to open it with "+<" or
    "+>" or "+>>" instead of with "<" or nothing.  If you intended only to
    write the file, use ">" or ">>".  See "open" in perlfunc.