Many thanks for your suggestion.
You all are correct, for ease of explanation I did not mention that the inputfile is a XML file.
Basically I have a hash which contains the tags of xml and their default values .
Now only those tags will be replaced which has a key in the hash. So the xml inputfile actually looks like :

**********************************
<user>
<default>dean</default>
</user>
<id>
<default>38339</default>
</id>
<workarea>
<default>/home/dean</deault>
</workare>
************************************

And lets assume the hash has 2 keys for id and workarea :

%hash = (id => 94848, workarea=> '/home/unix' )
So in this case while parsing the xml , I will only replace <default> values for <id> and <workarea>