=over

=item IO::Handle->output_field_separator( EXPR )

=item $OUTPUT_FIELD_SEPARATOR

=item $OFS

=item $,
X<$,> X<$OFS> X<$OUTPUT_FIELD_SEPARATOR>

The output field separator for the print operator.  If defined, this
value is printed between each of print's arguments.  Default is C<undef>.

You cannot call C<output_field_separator()> on a handle, only as a
static method.  See L<IO::Handle|IO::Handle>.

Mnemonic: what is printed when there is a "," in your print statement.

=back