It is fine to stay with array. Hope the following code helps:

Made a general purpose function to handle both replace and insert, and determines whether to insert or replace on its own:

use warnings;
use strict;

my @a = ("a=1", "b=2", "c=3");
print join(",", @a), "\n";
add("b", 22);#this replaces
print join(",", @a), "\n";
add("d", 45);#this inserts
print join(",", @a), "\n";

sub add {
    my ($key, $val) = @_;
    my $exists = 0;
    for (grep /^$key=/, @a) {
        $_ = "$key=$val";
        $exists = 1;    
    } 
    if (!$exists) {
        push @a, "$key=$val"
    }
}
[download]

Oh, COOL... That's great! I think I can make it work. But tomorrow. Not tonight. I have a headache, and it's screaming for Excedrin. ;) I'll let you know how it works out. THANK YOU!
Dev Goddess
Developer / Analyst / Criminal Mastermind

"Size doesn't matter. It's all about speed and performance."

I liked the idea of using a hash - it should be more efficient than comparing every key to the entire list every time for large numbers of additions/substitutions.

use warnings;
use strict;

my @tied_array = ("a=1", "b=2", "c=3");

my %compare_hash;
#a little extra overhead in the beginning
#could be done more elegantly with map???
for( my $i=0; $i<@tied_array; $i++) {
    my ($key, $val) = split /=/, $tied_array[$i];
    #the index must be associated with the key
    #so we know where it goes in the original array
    $compare_hash{$key} = [$i, $val];
}

print join(",", @tied_array), "\n";
update("b", 22);#this replaces
print join(",", @tied_array), "\n";
update("d", 45);#this inserts
print join(",", @tied_array), "\n";

sub update {
    my ($key, $val) = @_;

    #checking for the existence of a key in a hash
    #should be more efficient than grepping through
    #what could be a very large array
    if( defined $compare_hash{$key} ) {
        unless( $compare_hash{$key}[1] eq $val ) {
            $tied_array[$compare_hash{$key}[0]] = "$key=$val";
            #overhead required to maintain the comparison hash
            $compare_hash{$key}[1] = $val;
        }
    } else {
        push @tied_array, "$key=$val";
        #overhead required to maintain the comparison hash
        $compare_hash{$key} = [$#tied_array, $val];
    }
}
[download]

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Update

Fixed use of square braces instead of curly ones when accessing hashes (PHP is rotting my brain!).