my %hash;
# get data as hash
open FILE, $file or die $!\n;
while (<FILE>) {
    my ($key, $value) = split;
    $hash{$key} = $value;
}
close FILE;

my %changes = ( 'str1' => 'val1', 'str2' => 'val2' );

# change data in hash (or add new elements)
$hash{$_} = $changes{$_} for keys %changes

# overwrite old file with modified hash
open FILE, ">$file" or die $!;
print FILE "$_\t$hash{$_}\n" for keys %hash;
close FILE;