use strict;
use warnings;

my $filename = "inputfile.txt";
my $outputfile = $filename . "out";
my( $infh, $outfh );

open(  $infh, '<', $filename   ) or die $!;
open( $outfh, '>', $outputfile ) or die $!;

while( my $line = <$infh> ) {
    if( $line =~ /\bWilliam\b/i ) {
        print $outfh "Nick\n";
    }
    print $line;
}

close $infh  or die $!;
close $outfh or die $!;

rename $outputfile, $filename or die $!;