B::Deparse, use it

$ cat junk.pl
! defined and $_ = ' ' for @$row;

$ perl -MO=Deparse,-p junk.pl
;
(defined($_) or ($_ = ' ')) foreach (@$row);
junk.pl syntax OK
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I've never understood what this means '$_',

Then you haven't read perlintro, go read it now, and try out the examples.

Also see perlvar, it also explains $_

In other words, what does @$row mean or what is it accomplishing?

It is de-referencing, see references quick reference, basically

my @array = 1 ..2;
print "@array\n";
my $ref = \@array;
print "@$ref\n";
__END__
1 2
1 2
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