Your code should work already. I tried it and got the expected results.
use strict;
use warnings;

my $newUser ='$dummy';
while (<DATA>) {

    s/O_PREFIX=\w+/O_PREFIX=$newUser/;

    # s/(?<=O_PREFIX=)\w+/$newUser/;

    print;
}

__DATA__
if [ $YOU = 'frog' ]
   then
     export OUSER=TNN
     export CAR=SWs
     export O_PREFIX=ss
     export OS=corvette

fi

[download]

And the output -
if [ $YOU = 'frog' ]
   then
     export OUSER=TNN
     export CAR=SWs
     export O_PREFIX=dummy
     export OS=corvette

fi

[download]

In reply to Re: REGEX with input file by Roger
in thread REGEX with input file by samy1212

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