#!/usr/bin/perl

use strict;
use warnings;
use Tie::File;

my $file = 'test.ini';

tie my @lines, 'Tie::File', $file or die $!;
for my $line ( @lines ){
    if( $line =~ /^UPDLEVEL/ ){
        $line = "UPDLEVEL=neuer Wert\n";
        last;
    }
}
untie @lines;