As the only value of $row->{parentid} that could produce a hex value of 0x00000000 is a string of 4 ascii 0 (zero) bytes, then you could do

if ( $row->{parentid} =~ /^\0{4}$/ ) {
   # do something;
}
else {
   $parent_hex = '0x0' . unpack 'H*', $row->{parentid};
   # do something else
}
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