#!/usr/bin/perl
use strict ;
use warnings ;
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standard program startup $^I=".bak" ; Enables in-place editing. I.e, Changes will be made to the same file for files read through <>

@ARGV=(qw/abc.txt/) ;
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Makes <> read on abc.txt

while(<>) {
 s/trojan is like viruse/hello/g;
 print ;
}
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Replace "trojan is like viruse" with "hello". As in-place editing is enabled and print is not supplied an explicit filehandle, changes are made to the same file.