#!/usr/bin/perl

my @my_arr = ("test1", "test2", "test3", "test2", "test5", "test2", "test6");
my $what = 'test2';
my $then = 'best2';

my %index;
for ( my $i = 0; $i < @my_arr; $i++ ) {
	my $value = $my_arr[ $i ];
	$index{ $value } = $i if not exists $index{ $value };
}

if ( exists $index{ $what } ) {
	$my_arr[ $index{ $what } ] = $then;
}

print $_, "\n" for @my_arr;