LanX is right about $" being a standard variable and global. He is also right that its default value must have been changed somewhere (possibly in a module you use) to undef to raise the warning you are seeing.

You can "fix" the value of $" by:

local $" = ' ';

open ...
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which restores the default space used for $".

If you recently added use warnings to your script that would show a warning for a nasty condition that you previously weren't aware of. An array interpolated into as string behaves like:

my $arrayStr = join $", @{$table_A[$d]};
my $row1 = "$arrayStr";
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so the undef value in $" would have turned into an empty string and the array elements would have been stuck together instead of separated by single spaces.