#!/usr/bin/perl
open FH,'ent.txt';
@data=<FH>;
$text='&bush&amp;';

foreach $dat(@data){
$text =~ s/&/&amp;/g;
print $text;
}
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Maybe it would be a good exercise to go through the code you've posted and to explain what every line is supposed to do. Especially the intention of these two parts is important:

open FH,'ent.txt';
@data=<FH>;
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What does @data contain? Is data a good name for that?

foreach $dat(@data){
$text =~ s/&/&/g;
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1. Why are you iterating over @data?
2. What does change in the loop code for each iteration?
3. How does the code in the loop reflect the rules you set up for when to replace & with &?

What is the output you see, and what is the output you expect. How does the output you expect relate to the rules you've set up for when to replace &?

In answering these questions to yourself, you will find the steps you need to take to modify your program from a collection of lines into something that does what you want.