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IO::Handle->output_field_separator( EXPR )
$OUTPUT_FIELD_SEPARATOR
$OFS
$,

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

You cannot call output_field_separator() on a handle, only as a static method. See IO::Handle.

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