! defined and $_ = ' ' for @$row;
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Could also be written as:

not defined( $_ ) and $_ = ' ' for @$row;
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Or more simply in modern versions of Perl:

$_ //= ' ' for @$row;
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1) ! means "not" as in the not operator (see example above.)

2) You can also use it for filehandles '_' and subroutines '&_'.    It is just another valid character that can be used as variable names (see perlvar.)

3) $row contains a reference to an array and to dereference the array you put a '@' in front of $row.