#! perl
use strict;
use warnings;
use Tie::File;

my $filename = 'config.txt';

tie my @array, 'Tie::File',  $filename
    or die "Cannot tie file '$filename': $!";

for my $line (@array)
{
    my @fields = split /=/, $line;

    if ($fields[0] eq 'IPADDR')
    {
        $fields[1] = '1.2.3.4';
        $line = join '=', @fields;
        last;
    }
}

untie @array;