grep can help. It actually helps you in both replace and search (before you do insert). Read perlfunc for details.

use warnings;
use strict;

my @a = ("a=1", "b=2", "c=3");
for my $e (grep /^b=/, @a) {
    $e = "b=22";
}
print join(",", @a);

[download]

The other way is to use hash, as you are handling key-value pairs.

Update:

See my reply to devgoddess's reply. I made a general purpose sub there.

In reply to Re: Inserting lines in a file with Tie::File if lines don't exist by pg
in thread Inserting lines in a file with Tie::File if lines don't exist by devgoddess

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