• Foreach loops alias $_ to each element of the list in turn (unless told to use a different variable). Any change to $_ will change the element to which its aliased.
• and short-circuits, meaning it will only evaluate its RHS if its LHS is true.
• defined with no argument is defined($_).
• @$row is an array dereference. It means "the array referenced by $row".

These all do the same:

!defined and $_ = ' ' for @$row;
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for (@$row) {
   !defined and $_ = ' ';
}
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for (@$row) {
   if (!defined) {
      $_ = ' ';
   }
}
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for $_ (@$row) {
   if (!defined($_) {
      $_ = ' ';
   }
}
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for my $i (0..$#$row) {
   if (!defined($row->[$i]) {
      $row->[$i] = ' ';
   }
}
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In 5.10 and above, that could have been written

$_ //= '' for @$row;
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