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"
	}
}