A simplification of your code will fix it,

while(<MYFILE>){
    s/ORA3=\w+/ORA3=0/;
    print COPYFILE $_;
}
[download]

The space in your match was hurting you.

And then there is always the command-line edit-in-place version: $ perl -pi -e's/ORA3=\w+/ORA3=0/' file