use Tie::File;

my $file = 'filename';
my @tied_file;

my $idtag = 'idTag';
my $replacement = 'replacement line';

# Create the array @tied_file from $file
tie(@tied_file, 'Tie::File', $file);

# Preserve the index so we can work directly on lines of @tied_file
foreach my $i (0 .. $#tied_file) {

    if ($tied_file[$i] =~ /$idtag/) {

        # Do something to this line of @tied_file
        $tied_file[$i] = $replacement;

    }

}

untie(@tied_file);